Calendar reminder keeps popping up

I put in an appointment and 15 minutes (I set this in prefs) before it was due a reminder popped up, OK. No matter what I press though, dismiss all or the red cross the reminder keeps coming up after the due time saying the appointment is overdue by x minutes, how can I stop this reminder from popping up forever. What if I have dozens of old appointments will they all pop up saying overdue?

Having come back today after leaving testing for a couple of days it seems to be working OK without doing anything to the cache. I'll keep an eye on it and if I can recreate the problem I will try clearing the cache to see if that is a fix. Thanks